Fixation devices are used in many surgical procedures, for example to secure vertebral fractures during healing, and to fuse damaged vertebrae against movement. Such devices typically include a longitudinal support such as a rod or U-shaped member which is shaped to fit along the spinal column; a hook, clamp or screw type fastener which engages a vertebra; and a bracket or connector which interconnects the fastener to the longitudinal support. All three types of fasteners -hook, clamp, and screw--are widely used for fixation of the thoracic and lumbar regions (i.e., below the cervical vertebrae) where there are relatively large vertebral bone structures to carry the load. In those regions, fasteners of the hook and clamp types can be hooked over or engaged with posterior elements of the vertebrae or lateral projections. Large so-called "pedicle screws," which have coarse threads and are roughly similar to a lag screw are also used in the lumbar and thoracic regions. They extend from a bracket down the pedicle of the vertebra and into the vertebral body.
However, it is more difficult to attach hooks, clamps and screws in upper spine areas, that is, in the thoracic and cervical areas, because there the bone anatomy becomes progressively smaller. There is not adequate space or bone mass to secure pedicle screws, which are too large; and hooks sometimes tend to work loose in this area because of the hypermobility of the spine.
To achieve fixation in the cervical spine, the most common posteriorly used fixation devices are what are known as lateral mass plates. Screws are used to secure these but are much smaller than the pedicle screws used in lower thoracic and lumbar spine fixations. The screws are driven into the lateral masses of the vertebral body through holes in a plate which vertically bridges sequential vertebral segments. The locations in which these screws can be placed are limited so that each screw must be placed in a closely defined ideal position. The device must accommodate the requirements of mounting each screw in a precisely selected location, because the screws cannot be forced into any other position.
To date, however, there has been no practical way of coupling these types of screw fixations, that is, to couple a device of the type suitable for cervical spine fixation with a support that allows either a rod type of construct for posterior cervical fusion or extension of a thoracic fusion across the cervicothoracic junction. The same lack of inter-changeability between these two different types of constructs has also limited their application superiorly, that is, across the upper end of the spine, between the occipital bone of the skull and the cervical spine.
Thus there has been a need for a fixation device which will allow mating both lateral mass fixation screws and C1-C2 fixation screws with a rod-type support that can be combined with other types of fixation device at lower positions on the spine, for example, across the cervicothroracic junction.
In fixation devices of the screw type, the screw usually extends through a first opening in a bracket while the longitudinal support extends through a second opening in the bracket. U.S. Pat. Nos. 5,403,314 and 5,312,404 both disclose such apparatus. In those patents, once the bracket has been placed on the rod, it can be rotated about one axis only, i.e., circumferentially around the rod. The bracket cannot be rotated about an axis perpendicular to the rod. Thus the angulation of the axis of the bracket is fixed with respect to the axis of the support rod. This makes it difficult to "aim" a screw if it is to be secured into a small area of a cervical vertebra.
Another disadvantage of the fixation devices of those patents is that the physician has to determine in advance the number of fasteners and connectors to be used for a particular surgical procedure, before the rod or rods are fixed in position along the spinal column, because connectors could be attached to a rod only by sliding them in sequence along the length of the rod. After the rod was secured to the spinal column, an additional or intermediate connector could not be slid onto the rod without first removing at least one end of the rod from connection to the spine of the patient. If some fasteners were already secured to vertebrae and more were needed, they could be added only by removing the secured fasteners or the entire device from the patient, so that they could be slid along the rod to the desired location.
It has been an objective of this invention to provide a system which mates a screw fixation technique for the cervical spine with a rod-type support which allows fixation to be extended either superiorly or interiorly on the spine.
It has been a further object of this invention to provide a fixation device for cervical spine use, having a bracket that is rotatable about multiple axes, so that the opening through which the fastener extends can be positioned in a wider range of orientations in relation to the rod, for precise screw orientation.
Another object of this invention has been to provide a bracket that may be mounted to a rod which has itself already been secured to the spinal column.